ASCII Interface - Atmega328p

This is is a key scanner module with parallel output, and optional serial output, supporting keyboards of up to 16 rows X 8 columns.

The ATMega 328P was selected for the sole reason that it is an architecture familiar to many hobbyists, with an accessible programming environment and ecosystem, in a DIP format that fits the retro look and is easy to solder for most hobbyists. All of the ICs, other than the microcontroller, are only present to compensate for the limited number of GPIO lines on this small 28-pin microcontroller. Using a bigger chip would greatly simplify the hardware and even slightly simplify the hardware layer of the firmware.

Overview

The keyboard rows are driven by a pair of 74LS138 decoders, allowing 4 GPIO lines to drive 16 rows.
The columns are read in via an 8-bit shift register, controlled by 3 GPIO lines.
One 8-bit port is used for the parallel ASCII output.
Two GPIO lines are used to generate special outputs to the host. These could be RESET, BREAK, CLEAR, etc.
One GPIO line is used to control a keyboard LED.
Three of the ISP lines are also used as outputs to controll two more Keyboard LEDs and one more "special" output line.
The top two parallel I/O bits can also be configured as UART I/O. This could be used to provide serial output instead or parallel output, to support a bootloader, or even to accept a serial input stream from another computer to send to the host as parallel ASCII data. This last application would require careful timing to avoid conflicts.

Assembly Notes

The minimum functioning circuit includes

the Microcontroller (U1)
the 74LS166 shift register (U2)
one 74LS138 decoder (U3).
The resistor network RN1
The two resistors R3 and R4. If you are using the keyboard in only a parallel configuration or only a serial configuration, then you can jumper these resistors with a piece of wire.
Any connectors required.

Optional components

Diodes D1-D16

These diodes are intended to mitigate ghosting when used with a keyboard matrix that does not have any diodes installed. These provide one diode per row. If you are attaching a keyboard with no diodes, then you only need to populate the diodes corresponding to rows on the keyboard. If the keyboard has 8 rows, then you may want to install 8 diodes corresponding to those rows.

Note that the footprints for these diodes include a copper jumper on the TOP copper layer. If you install any of these diodes, you should cut the jumpers for those diodes. Otherwise the diodes do nothing.

DIP switch and associated diodes

The DIP switch is optional. If you don't want to be able to set options via the DIP switches, you can set all your preferences in the firmware, or just accept the default behavior, and skip the DIP switch and Diodes D17-D20 and D24-D27.

Second 74LS138 multiplexer (U4)

Only needed if you have more than 8 rows.

The 74LS07 hex buffer (U5)

Only needed if you are using LED2 or LED3 on the keyboard, or the OUT2 open collector output.

Diodes D21, D22, D23

Some keyboards may not wire all keys into the matrix. For example, some other interface modules may assign special hardware functions to certain keys. Those keys are brought directly to the keyboard connector. For the classic keyboard, the POWER key, the '@' key, and the RUBOUT key are not directly wired. This module wires them into the matrix via D21, D22, and D23, respectively. For keyboards with no separately wired keys, these diodes may be omitted.

Resistors R1, R2, R5

These resistors provide current limiting for up to three keyboard LEDs. If no keyboard LEDs are installed, then these resistors can be ommitted.

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